After a Bleed, I've no stopping power.. What's wrong?!

Hiya folks, newbie here so please excuse my terminology.

Problem : After a brake bleed, my brake lever is hard/good/tight, but I now donít have much stopping power, and sometimes the brake squeaks.

Background: I have just bought a 2nd hand bike with Shimano BR-M445 hydraulic disc brakes.
The rear brake lever had a lot of Ďtravelí in it before the brakes gripped, so I decide to have a try at bleeding.

I removed the brake pads, used spacers, and did a bleed BUT then stupidly took out the spacers and used the brake lever and the piston popped out (along with some oil). Oops!!!
So, I fixed that, did a re-bleed, and it felt good to the touch (brake levers seemed perfect), but this morning on its first ride, when I brake, I donít skid/stop but instead I slow down and the pads just donít seem to grip any more no matter how much pressure I have on the lever. Also itís squeaks a bit.

Could you offer any suggestions as to what the problem is. Or how I can fix.

I was thinking maybe the same as I used the old pads, but, when i did the bleed i had removed them and used spacers and did my best not to get them mucky.
Could this still be the case? are the pads super sensitive to oil?